Spanish Double Object Pronouns?

Could someone help me understand how to do this? So I'm giving a question in spanish and I have to answer it using double object pronouns. The example giving was... ¿Quién te compró esos zapatos? (mi tío) Mi tío me los compró. But I still don't understand how to solve this! Here is one of the... show more Could someone help me understand how to do this?

So I'm giving a question in spanish and I have to answer it using double object pronouns.

The example giving was...
¿Quién te compró esos zapatos? (mi tío)
Mi tío me los compró.

But I still don't understand how to solve this!

Here is one of the questions...
¿A quiénes les escribiste las cartas? (a ellos)

Any suggestion on how to do this?
Update: For the first question, which says...
¿A quiénes les escribiste las cartas? (a ellos)

I entered in...
Se las escribiste.

But it said that the answer was wrong!
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